Abstract:

The aim of this study was to assess the potential nutritive value for sheep, of two drought tolerant leguminous shrubs (Cassia sturtii and Sutherlandia microphylla) in terms of chemical composition, degradation parameters, digestibility, rumen fermentation parameters, intake, microbial nitrogen synthesis and nitrogen balance as well as the rumen kinetics when compared to that of Medicago sativa. The crude ash concentration of all three forages differs, with S. microphylla and C. sturtii lower than M. sativa. M. sativa has a crude ash concentration almost twice the amount of both S. microphylla and C. sturtii. Wilcock et al., (2004) reported ash values for C. sturtii stems and leaves of 53 and 73 g/kg and that of S. microphylla at 25 and 64g/kg respectively. Values for C. sturtii are lower while those of S. microphylla compare well to the average of the whole plant. The mean CP and CF concentration differed between species with C. sturtii having the lowest CP and M. sativa the highest. S. microphylla had the highest CF while M. sativa had the lowest. The NDF and ADF levels of the samples varied between all three species with S. microphylla being the highest and M. sativa the lowest. Values for C. sturtii were in between those of the two other forages. The ADL concentration of S. microphylla was higher than both C. sturtii and M. sativa. The degree of lignification in C. sturtii was high (23.8% of NDF was ADL). The degree of lignification of S. microphylla was 26.8%, which is higher than that of C. sturtii, while M. sativa is the same as C. sturtii. The calcium concentrations of C. sturtii and M. sativa are similar and have a higher concentration than S. microphylla. M. sativa and C. sturtii had a higher phosphorus concentration than S. microphylla. With respect to magnesium (Mg), C. sturtii and M. sativa have a similar composition while S. microphylla has a lower concentration. The iron concentration of all three plants differs, with M. sativa having the lowest concentration and C. sturtii the highest. The copper concentrations in M. sativa and C. sturtii were similar, while that of S. microphylla was slightly lower. The zinc concentrations in M. sativa and C. sturtii were similar, while that of S. microphylla was slightly higher. Manganese concentration of all three species differs, with C sturtii being the lowest and S. microphylla the highest. The plants from this trial were analysed for selenium but none or very insignificant levels were found and were not worth reporting. The apparent DM digestibility of S. microphylla is significantly lower than M. sativa while it did not differ significantly from C. sturtii. C. sturtii did not differ significantly from both M. sativa and S. microphylla. The CP digestibility of all three species did not differ significantly, however that of M. sativa is numerically higher. With regards to the apparent NDF digestibility, C. sturtii and S. microphylla differ significantly to M. sativa with lower NDF digestibility values. The apparent OM digestibility followed the same trend as that of apparent DM digestibility. The average intake was very different between species, with C. sturtii being the lowest and M. sativa the highest. The animals consuming either C. sturtii or S. microphylla tended to lose body weight during the experimental period, while those eating M. sativa gained body weight. Voluntary intake parameters of C. sturtii and S. microphylla were lower and differed significantly between M. sativa. The DM intake of M. sativa was higher than both C. sturtii and S. microphylla. The ME was the highest for M. sativa while S. microphylla was significantly different and had the lowest value. C. sturtii had an ME value similar to both M. sativa and S. microphylla. The ME intake of S. microphylla was 2.89 MJ/day compared to that of M. sativa of 8.57 MJ/day. Rumen NH3-N concentrations of C. sturtii were the lowest and differed significantly from S. microphylla and M. sativa. Sheep receiving C. sturtii had the lowest total rumen VFA concentration and was significantly different from M. sativa which had the highest value. S. microphylla had a similar total VFA concentration to both C. sturtii and M. sativa. C. sturtii had the lowest proportion of acetate but did not differ significantly compared to S. microphylla, while both were significantly different to M. sativa, which had the highest value. The propionate concentration for all three forages did not differ significantly. S. microphylla had the highest fibre concentration, therefore leading to higher acetate concentrations than C. sturtii but not higher than M. sativa, suggesting the fibre of S. microphylla is less digestible. This is supported by the low apparent NDF digestibility for S. microphylla. Nitrogen intake was highest for M. sativa and was significantly different from C. sturtii and S. microphylla. The same trend followed for faecal and urinary nitrogen output as well as nitrogen retention. The nitrogen retention for all species was positive with C. sturtii being the lowest. These values compare well to the CP content of the three forages with C. sturtii the lowest and M. sativa the highest concentration. The daily urinary allantoin elimination did not differ between C. sturtii and S. microphylla but was significantly different and higher for M. sativa. The amount of microbial nitrogen supplied to the animal (g/day and g/kg DOMI) followed the same trend as allantoin. M. sativa had significantly higher a-values (soluble fraction) for both DM and NDF degradation compared to the two shrub species at a rate constant of 0.02/h. C. sturtii had a higher b-value (potentially degradable fraction) for DM degradation compared to S. microphylla which shows that S. microphylla DM component was most readily soluble. For NDF, however, the b-values didn’t differ among the species. Species had also no effect on the c-values (rate of degradation of the potentially degradable fraction b) of both DM and NDF. Therefore all species appear to have a similar potential source of energy for use by micro-organisms in the rumen. Effective DM degradability of C. sturtii and S. microphylla was similar while that of M. sativa was significantly higher. The effective NDF degradability for C. sturtii and S. microphylla was similar and M. sativa again had a significantly higher NDF degradability. The rumen DM degradability for all three species showed a similar trend but much higher values than the apparent DM digestibility. The rumen NDF degradability values were almost identical to those reported for apparent NDF digestibility. The rate of intake and rate of digestion for C. sturtii and S. microphylla did not differ significantly, while that of M. sativa was the highest and significantly different. The rate of passage for all three species was similar. The percent NDF digested in the rumen differed significantly between all three species with C. sturtii being the lowest and M. sativa the highest. The percent NDF passing from the rumen also differed significantly between all three species, however this time C. sturtii being the highest and M. sativa the lowest, which corresponds well to the values for NDF digested in the rumen. It is concluded that C. sturtii and S. microphylla are of a slightly lower nutritional value for sheep than M. sativa. If these two leguminous fodder species were to be used as maintenance feed, some other supporting source of energy would need to be supplied in order for these sheep to be maintained over a long period. The negative effect of all fibre related parameters (CF, NDF, ADF and ADL) in C. sturtii and S. microphylla, reduced digestibility as well as intake, leading to a forage of lower nutrient value as compared to M. sativa. The effect of anti-nutritional factors present in C. sturtii and S. microphylla on the digestibility of forages and nutrient contribution from forages needs to be studied to determine if these play a role in reducing the nutritional value. Copyright